Process for forming nanoparticles

ABSTRACT

A metal or alloy nanoparticle is provided which exhibits hysteresis at room temperature having a carbon coating. The nanoparticle has a diameter in the range of approximately 0.5 to 50 nm, and may be crystalline or amorphous. The metal, alloy, or metal carbide nanoparticle is formed by preparing graphite rods which are packed with the magnetic metal or alloy or an oxide of the metal or alloy. The packed graphite rods are subjected to a carbon arc discharge to produce soot containing metal, alloy, or metal carbide nanoparticles and non-magnetic species. The soot is subsequently subjected to a magnetic field gradient to separate the metal, alloy, or metal carbide nanoparticles from the non-magnetic species.

This application is a divisional application of the United States patentapplication Ser. No. 08/265,008 filed Jun. 24, 1994, U.S. Pat. No.5,549,973, which is a continuation-in-part application of United Statespatent application Ser. No. 08/085,298, filed Jun. 30, 1993, now U.S.Pat No. 5,456,986 issued Oct. 10, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of metal, alloy, or metalcarbide compounds and carbon-coated magnetic metal alloy or metalcarbide compounds. Particularly, the present invention relates to thefield of carbon-coated metal, alloy, or metal carbide nanoparticles andthe methods for preparing the same. Nanoparticles include crystalline oramorphous particles 0.5 to 50 nanometers in diameter and nanotubes up to1 centimeter long and 0.5 to 50 nanometers in diameter.

2. Description of the Prior Art

Small magnetic particles have many applications. Such articles are usedas toner in xerography, in ferrofluid vacuum seals, in nuclear magneticresonance imaging as contrast agents, and in magnetic data storage.These magnetic particles are typically micron-sized in diameter orlarger. The large size of these particles renders them less thansatisfactory for several specialized applications.

If the magnetic particles were smaller, cost reduction by reducing thenumber of processing steps would be achieved in xerographicapplications. In ferrofluid applications, the enhanced solubility due tocarbon coating provided by smaller particles may be advantageous. Inmagnetic data storage, high density may be enhanced by using smallerparticles. Moreover, in magnetic ink applications, the carbon coatingand ability to disperse the nanoparticles in aqueous solutions mayprovide advantages for wetting and coating. Consequently, there is apotential need for sub-micron sized metal, alloy, or metal carbideparticles and a method for producing bulk amounts of these particles ina high yield process. In order to improve operation in magnetic datastorage applications, it is desirable for the magnetic particles toexhibit hysteresis. Even more desirable is the exhibition of hysteresisat room temperatures.

Recently, there has been increased investigation regarding theKratschmer-Huffman carbon arc method of preparing fullerenes, or smallhollow carbon clusters. These fullerenes are typically in the order of 1nm in diameter. Recently, it has further been discovered that thesehollow carbon clusters can be filled with metal ions. This can beaccomplished by drilling out the graphite rods and packing them with amixture of metal oxide powder and graphite cement before generating thesoot by the carbon arc. Rodney S. Ruoff, Donald C. Lorents, Bryan Chan,Ripudaman Malhotra, and Shekhar Subramoney, Science, Vol. 259, p. 346(1993) discussed the production of 20-40 nm diameter carbon-coatedlanthanum carbide nanocrystallites by this method. Similar results werereported by Masato Tomita, Yahachi Saito and Takayoshi Hayashi in Jpn.J. Appl. Phys., Vol. 32, p. 280 (1993).

The carbon arc method of preparing lanthanum carbide nanocrystallitesdiscussed above generates fullerenes and graphitic soot in addition tothe lanthanum carbide nanocrystallites. In order to be useful, a meansof separating the nanocrystallites is essential. So far, no chemicalmethods have been found to be successful in separating macroscopicamounts of nanoparticles from graphitic soot and fullerenes. Suchseparation processes are rendered extremely important when the yieldsachieved for the nanoparticles is in the order of ten percent or less ofthe soot. Accordingly, there is a need for a method to separate metal,alloy, or metal carbide nanoparticles from graphitic soot.

SUMMARY OF THE INVENTION

In Application Ser. No. 08/085,298, filed Jun. 30, 1993, a modificationof the Kratschmer-Huffman carbon arc method was used to form carboncoated nanoparticles having a diameter in the range of approximately 0.5to 50 nm. It has now been found that if a magnetic rare earth metal,alloy or metal or alloy oxide is packed into a graphite rod andsubsequently subjected to a carbon arc discharge, soot containing metal,alloy, or metal carbide nanoparticles and non-magnetic species isformed. The metal, alloy, or metal carbide nanoparticles can beseparated from the soot by subjecting the soot to a magnetic fieldgradient.

In the magnetic separation step, the nanoparticle-containing soot isground to a fine powder and then dropped down an electrically groundedmetal tube through a magnetic field gradient created by a pair of strongmagnets. Non-magnetic material passes through the tube, but magneticcomponents are suspended if the field gradient force exceeds thegravitational force. When the apparatus is moved away from the magnets,the magnetic material is released into its own separate collectioncontainer. This process can be used to separate paramagnetic orferromagnetic species from non-magnetic components of the soot producedby the carbon arc discharge process.

Theories of monodomain magnetic particles predict that the blockingtemperature, i.e., the temperature above which metastable hystereticbehavior is absent, depends on the product of the particle volume andthe magnetocrystalline anisotropy constant for the material, K. Thealloys manganese aluminum carbide (Mn₃ AlC), τ-phase manganese aluminide(MnAl) and several samarium cobalt (SmCo_(x)) phases are ferromagneticin bulk form and have large anisotropy constants. The magnetization as afunction of the applied field and temperature for the different alloynanoparticles indicates that monodomain magnetic particles of thesealloys exhibit room temperature hysteresis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SQUID magnetometer measurement of M(H,T) for a 30 mgnanocrystalline manganese-aluminum-carbide specimen formed in accordancewith the present invention.

FIG. 2 is a plot of the measured coercivity as a function of T^(M) forthe specimen used in FIG. 1.

FIG. 3 is a SQUID magnetometer measurement of M(H,T) for a samariumcobalt powder specimen formed in accordance with the present invention.

FIG. 4 is a curve showing the coercivity as a function of T^(M) for thespecimen used in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A process based on the Kratschmer-Huffman carbon arc method of preparingfullerenes can be used to generate carbon-coated metal, alloy, or metalcarbide nanoparticles. When combined with a magnetic field gradientseparation technique, bulk amounts of these nanoparticles can beisolated.

If graphite rods which are packed with a magnetic rare earth metal,alloy or metal or alloy oxide are subsequently subjected to a carbon arcdischarge, the soot produced by the Kratschmer-Huffman carbon arcprocess contains metal, alloy, or metal carbide nanoparticles andnon-magnetic species. The graphite anode is hollowed out and packed witha mixture of metal, alloy, or metal or alloy oxide and graphite cement.In the arc, metal-containing clusters form. The cluster stoichiometrydepends on the chemistry between the metal atoms, carbon, and oxygen.The clusters diffuse until they are deposited on a surface, either thehigh temperature cathode or the room temperature walls of the reactor.The nanocrystalline phases produced depend on the surface temperatureand the cooling rate of the clusters determined by their diffusion pathand the amount of helium buffer gas. This method is therefore useful inpreparing metastable phases. The carbon coating arises when theparticles cool. Since graphite melts at a higher temperature than themetals or metal carbides, phase segregation occurs in the coolingnanoparticle, forming a graphitic shell. The coating in some casesprevents degradation of air or water-sensitive compounds, a problemencountered with important magnetic materials such as Nd₂ Fe₁₄ B.

A magnetic field gradient can be used to separate the metal, alloy, ormetal carbide nanoparticles from the non-magnetic species included inthe soot. Preferably, the soot produced by the carbon arc dischargemethod is further ground to a fine powder before being subjected to themagnetic field gradient. If the magnetic field gradient force is greaterthan the gravitational force, the magnetic nanoparticles will besuspended by the magnets in the separator tube whereas the non-magneticspecies will pass through. This magnetic gradient separation techniqueremoves non-magnetic byproducts of the carbon arc discharge process andenhances the magnetic response of the isolated material. This separationprocess separates all paramagnetic or ferromagnetic components from theremaining soot.

If the particles are formed from a ferromagnetic material, their size issmall enough to support only a single magnetic domain. These particlesare said to be superparamagnetic, and all the atomic spins align toyield a large particle moment. The particle moment rotates not throughdomain wall motion, but by rotating all the atomic spins together.Theories of superparamagnetism show that the temperature dependence ofthe coercivity, H_(c), for a spherical particle with a single magneticdomain is govern by the equation:

    H.sub.c =H.sub.ci  1-(T/T.sub.B !.sup.1/2,                 (1)

where H_(ci) is the coercivity at 0° K. and T_(B) is the blockingtemperature. Above the blocking temperature, the particle's magneticmoment can flip due to thermal fluctuations during the time it takes tomeasure the bulk magnetization, which can be on the order of an hour.Above this temperature, the particle is superparamagnetic and nohysteresis is observed in the time frame of the measurement. Below theblocking temperature, the particle doesn't have enough thermal energy tospontaneously flip its magnetic moment in this time frame, and nonzerocoercivity is expected, with the temperature dependence given byequation 1. The energy, E, of the single domain spherical magneticparticle, is:

    E=V K sin.sup.2 θH M cos φ!,                     (2)

where V is the particle volume, K is the magnetocrystalline anisotropy,H is the applied field, M is the particle magnetic moment, θ is theangle between the particle moment and the nearest easy axis direction,and φ is the angle between the applied field and the particle moment. Inthe absence of an applied field, the particle moment will lie along aneasy axis to minimize the energy; but in a strong applied field, themoment will align with H. In order to rotate the moment to align withthe field, the particle must overcome an energy barrier on the order ofKV. Above the blocking temperature, the particle's magnetic moment mayswitch directions spontaneously due to thermal fluctuations. Theblocking temperature may be related to the inverse time for measuringthe magnetization, ω; the attempt rate to surmount the barrier, ω₀,which is on the order of the precession frequency of the moment; and theheight of the barrier, KV, according to the following relationship:

    ω=ω.sub.0 exp  -KV/kT.sub.B !.                 (3)

Fine particle magnets may have hysteresis up to a critical temperature,just like their bulk counterparts. However, the physics responsible forthe hysteretic behavior is quite different. For monodomain particles,the comparison between the thermal energy and the anisotropy energy isimportant, while for bulk ferromagnets the comparison between thethermal energy and the exchange energy determines the magnetic behavior.

The present invention can be more readily understood in connection withthe following examples wherein alloys having large magnetocystallineanistropy are examined.

EXAMPLE 1

In earlier preparations of elemental and carbide nanocrystals, thereactor conditions were adjusted to control the specific phases made. Inorder to obtain microscopic chemical homogeneity in the carbon arc, aMn-Al-O spinel phase starting material was prepared. Stoichiometricquantities of Mn₂ O₃ and Al₂ O₃ were mixed, sintered at temperatures onthe spinel phase field and quenched to retain the spinel phase, whichwas characterized by X-ray diffraction and TEM analysis. The spinelphase of variable composition is identified using a Vegard's lawdependence of the lattice constant on Mn:Al ratios between the two endcompositions. Mn-Al-O was a 2:1 ration was packed in graphite rods usedin the carbon arc to generate Mn-Al-C nanoparticles.

Based on the Mn-Al-C phase diagram, only 1-2% carbon dissolves into thealloy, and we anticipated that the metastable phase analogous to theτ-phase of MnAl would also be ferromagnetic. The only otherferromagnetic phase of Mn-Al-C was Mn₃ AlC, which has a Curietemperature of 15° C. Since the τ-phase is the only known ferromagneticphase of Mn-Al-C at room temperature, raw soot from different parts ofthe reactor was tested with a Nd₂ Fe₁₄ B magnet to quickly optimize theproduction of this ferromagnetic phase. The best arc conditions werefound to be 100 A and 40 V with a 1 mm gap spacing. This led to abundantferromagnetic material in the pancake region of the deposit which formson the cathode. Based on a growth model for the nanocrystals made in acarbon arc, both the temperature of the surface and the cooling rate ofthe alloy cluster before it deposits determines the nanocrystal phaseproduced. Here the temperature of the cathode is believed to be slightlycooler in the pancake region than in the central core deposit, but themain factor is the difference in the cooling rate of a cluster. Bulkτ-MnAl has been made by melt cooling with a cooling rate of 10⁵ -10⁸ °C./s, of the ε phase, followed by annealing.

The phases present in the sample were determined by several methods.Structural characterization by X-ray diffraction (XRD) indicated thepresence of Mn₃ AlC, γ-Mn, and graphite, but did not show peakscorresponding to the τ-phase. Calibration of the X-ray apparatusindicated that the maximum abundance of the τ-phase without being ableto detect it would be on the order of 4 weight percent. Energydispersive spectroscopy (EDS) was used to determine the Mn:Al relativeabundance. Here the electron beam was large enough to excite transitionsin several nanocrystals simultaneously. The fact that the ratio rangedfrom 3:1 to 4:1 over a wide area is consistent with the predominantspecies seen by XRD. Based on the response to the magnet at roomtemperature, at least some τ-MnAl-C is present in our samples. However,to test the evidence that a large amount of Mn₃ AlC was also present, asample was placed in a freezer well below the Curie temperature for thisphase. It was found that cooling significantly increased the magneticresponse.

High resolution TEM showed the particle shape, size, and sizedistribution. All particles were found to have carbon coatings, and theencapsulated nanoparticles were crystalline. An unusual feature was thepresence of elongated as well as spherical particles, but the phasescorresponding the different shapes were not assigned. In the elongatedparticles, typical aspect ratios were on the order of three to one. Ifthese particles are ferromagnetic, than shape anisotropy as well asmagnetocrystalline anisotropy would play a role in the magneticbehavior. Both the size (average diameter approximately 20 nm) and thesize distribution were similar to that seen in metal and metal carbidematerials made by the carbon arc process.

The magnetization as a function of the applied field and temperature wasdetermined using SQUID magnetometry. For SQUID measurements,approximately 30 mg of platelets from the Mn-Al-C sample wereimmobilized, so that the nanoparticles would not move in the appliedmagnetic field. Magnetization curves were measured between 5° and 200°K. and 0 to ±5 T. Hysteresis was observed at all temperatures withinthis range. The shape of the hysteresis loop in FIG. 1 is indicative oftwo ferromagnetic contributions with different coercivities, most likelyMn₃ AlC and τ-MnAl. The measured coercivity was plotted as function ofT^(1/2) in FIG. 2, clearly indicating a departure from the behaviorpredicated by equation 1. This difference is believed to be due to thepresence of multiple components. In this case the saturationmagnetization M_(sat), would equal the sum of the two contributions,normalized for relative abundance. The measured coercivity would beintermediate between those for the two components, but the specificvalue would depend on the shape of the contributing hysteresis loops.The slope of any fit to the date in FIG. 2 cannot be interpreted interms of a blocking temperature. However, extrapolation of the datasuggests that hysteresis would be present at room temperature, a noveltyfor monodomain magnetic particles.

EXAMPLE 2

Samarium cobalt has multiple stable ferromagnetic phases. In preparingalloy nanocrystals of samarium cobalt in the carbon arc, metallic Sm₂Co₇ powder, rather than an oxide material, was used. A strong magneticresponse was observed with a Nd₂ Fe₁₄ B magnet in material from allparts of the reactor. Data was taken from the pancake region of thecathode deposit, the same region studied for Mn-Al-C.

Structural characterization by XRD revealed the presence of SmCo₅, Sm₅Co₂ fcc Co, and graphite, but not SM₂ Co₇ or Sm₂ C₁₇, or samariumcarbide phases. EDS indicated a Sm:Co ratio of approximately 1:2 in thesample. The difference between this ratio and that in the startingmaterial may be due to the relatively high vapor pressure of Sm. TEMshowed that most particles were approximately spherical, with an averagesize on 20 nm.

In general, the Curie temperature rises with the proportion of cobalt.While the cobalt Curie temperature was above the detection range, it wasexpected that Sm₂ Co₇ (420° C.) and SmCo₅ (710° C.) would be observable,even with small increases due to the addition of carbon. However, TMAshowed no transitions between 25° C. and 875° C. The addition of carbonhas also been shown to increase the Curie temperature by over 260° C. inSm₂ Co₁₇ cobalt magnets, and a similar effect may be occurring in SmCo₅.

SQUID magnetization measurements were made on powder samples immobilizedin epoxy. The hysteresis loops presented in FIG. 3 show a morecharacteristic shape than for Mn-Al-C, and nonzero coercivity wasdemonstrated above room temperature. Fitting the temperature dependenceof the coercivity to equation 1 as shown in FIG. 4 yielded a blockingtemperature of approximately 3800° K., far in excess of the Curietemperature for any samarium cobalt alloy.

The model used to understand the loss of hysteresis above the blockingtemperature breaks down above ^(T) c where the exchange coupling betweenspins on neighboring atoms is disrupted by thermal fluctuations and theparticle ceases to have a giant magnetic moment. Above ^(T) c,regardless of the calculated blocking temperature, hysteresis will ceaseto exist.

The phenomenon of nonzero coercivity in monodomain ferromagnets at andabove room temperature is real and has potential significance forparticulate recording media. Small ferromagnetic particles are commonlyused in magnetic recording tapes, but these particles are typically muchlarger than those discussed in this article. Larger particle sizes havepreviously been used in order to obtain stability against thermalfluctuations and small changes in the applied field. Stable alloy magnetnanocrystals, such as the SmCo_(x) C! particles prepared in a carbonarc, meet the stability requirements with smaller sizes, and aretherefore a possibility for higher density data storage medium.

In the foregoing specification certain preferred practices andembodiments of this invention have been set out, however, it will beunderstood that the invention may be otherwise embodied within the scopeof the following claims.

We claim:
 1. A method for forming a metal or alloy nanoparticlecomprising the steps of:a. preparing a graphite rod, said graphite rodbeing packed with said metal or alloy or an oxide of said metal oralloy; b. subjecting said packed graphite rod to a carbon arc dischargeto produce soot containing magnetic metal alloy nanoparticles andnon-magnetic species; and c. applying a magnetic field gradient to saidsoot to separate said magnetic metal alloy nanoparticles from saidnon-magnetic species.
 2. The method of claim 1 wherein said nanoparticleis one of a ferromagnetic and paramagnetic compound.
 3. The method ofclaim 2 wherein said nanoparticle exhibits hysteresis at roomtemperature.
 4. The method of claim 3 wherein said metal alloy issamarium cobalt.
 5. The method of claim 4 wherein said samarium cobaltis in the form of at least one of SmCo₅, Sm₂ Co₇, and Sm₂ Co₁₇.
 6. Themethod of claim 3 wherein said metal alloy is a manganese-aluminumcompound.
 7. The method of claim 6 wherein said manganese-aluminumcompound is manganese-aluminum-carbide.
 8. A method for separatingcarbon coated magnetic nanoparticles comprising:forming a mixture ofcarbon coated magnetic and nonmagnetic nanoparticles by carbon arcdischarge; creating a magnetic field gradient having a magnetic forcesufficient to suspend the magnetic nanoparticles when the mixture ispassed through the magnetic field gradient; passing the mixture throughthe magnetic field gradient to suspend the magnetic nanoparticles andseparate the magnetic nanoparticles from the mixture; and, removing themixture that passed through the magnetic field gradient.
 9. The methodof claim 8, further comprising:releasing the suspended magneticnanoparticles from the magnetic field gradient; and, collecting thereleased magnetic nanoparticles.
 10. The method of claim 9, furthercomprising repeating said method of claim 8 with the magneticnanoparticles that were suspended and released from the magnetic fieldgradient and collected.
 11. The method of claim 8, wherein:said creatinga magnetic field gradient comprises positioning a pair of strong magnetsnear a separator to create a magnetic field gradient within theseparator; and, said step of passing further comprises passing themixture through the separator.
 12. The method of claim 11, furthercomprising separating the magnets and the separator to release thesuspended magnetic nanoparticles.
 13. The method of claim 8, wherein themagnetic nanoparticles have a magnetic moment, and said method furthercomprises varying the magnetic field gradient to segregate the magneticnanoparticles according to the magnetic moment.
 14. The method of claim8, wherein said passing comprises dropping the mixture such that agravitational force will act on the mixture and pass the mixture throughthe magnetic field gradient.
 15. The method of claim 8, wherein saidmagnetic nanoparticles exhibit hysteresis at room temperature.
 16. Amethod of producing separated carbon-coated magnetic nanoparticles,comprising the steps of:subjecting a magnetic metal, alloy or an oxideof a magnetic metal or alloy to a carbon coating process to form amixture of carbon coated magnetic nanoparticles and nonmagneticmaterial; creating a magnetic field gradient having a magnetic forcesufficient to suspend the carbon coated magnetic nanoparticles when themixture is passed through the magnetic field gradient; passing themixture through the magnetic field gradient to suspend the carbon coatedmagnetic nanoparticles and separate the carbon coated magneticnanoparticles from the mixture; and, removing the mixture that passedthrough the magnetic field gradient.
 17. The method of claim 16, whereinsaid step of subjecting further comprises the step of grinding themixture to form a fine powder prior to said step of passing the mixturethrough the magnetic gradient field.
 18. The method of claim 16, whereinthe suspended carbon coated magnetic nanoparticles contain an amount offullerenes, and said method further comprises the steps of:releasing thesuspended carbon coated magnetic nanoparticles from the magnetic fieldgradient; collecting the released carbon coated magnetic nanoparticles;and, extracting the fullerenes from the collected carbon coated magneticnanoparticles.
 19. The method of claim 16, wherein the carbon coatedmagnetic nanoparticles are selected from the group consisting ofparamagnetic and ferromagnetic compounds.
 20. The method of claim 19,wherein the carbon coated magnetic nanoparticles are selected from thegroup consisting of the transition metals.
 21. The method of claim 20,wherein the carbon coated magnetic nanoparticles are selected from thegroup consisting of iron, cobalt, nickel, and manganese bismuth.
 22. Themethod of claim 21, wherein the carbon coated magnetic nanoparticles aresamarium cobalt.
 23. The method of claim 22, wherein samarium cobalt isof the form SmCo₅, Sm₂ Co₇, and Sm₂ Co₁₇.
 24. The method of claim 19,wherein the carbon coated magnetic nanoparticles are manganese-aluminumcompounds.
 25. The method of claim 24, wherein the manganese-aluminumcompounds are manganese-aluminum-carbides.
 26. The method of claim 19,wherein the carbon coated magnetic nanoparticles are selected from thegroup consisting of the rare earth metals except for lanthanum, lutetiumand promethium.